1. Field of the Invention
The present invention primarily relates to a rotary diaphragm press of a rotary drum type, which aims at continuous dehydration of sludge discharged from an industrial waste water treatment plant, a sewage treatment plant and the like.
2. Description of the Prior Art
Frequently, such sludge contains aluminium hydroxide as the main component or is excess activated sludge produced in the process of biological treatment. Generally, a dewatered cake obtained from such sludge is still, for the most part, water and thus involves difficulties in conveying it to a remote place for discharge or in drying, burning it up thereby incurring much expense for handling, transportation thereof or requiring much fuel consumption. For this reason, it is desirable to reduce the percentage of water contained in the dewatered cake as much as possible in the process of dehydration.
Various techniques have been employed in the dehydrating of sludge. Among them, a pressure squeeze technique which is typified by a filter press is far superior in reduction of the percentage of water contained in the dewatered cake. Disadvantages derived from this technique are that due to its batch operation much squeezing time is required to make the cake in large thickness for the purpose of securing a treated amount and for proving exfoliation of the cake, and also many accessory installations are required for the batch operation and it is difficult to render the dehydration process continuous.
On the other hand, a continuous dehydrator which relies on vacuum or centrifugal technique has been widely used but its use is limited to an attainable percentage of water content when the above-mentioned sludge is treated.
There has been proposed a belt pressure dehydrator as described in Japanese Utility Model Publication No. 25717/72.
In view of the fact that a reduction in the percentage of water contained in the cake depends upon the squeeze pressure and the time for squeeze, a belt pressure dehydrator of this class is designed so as not to apply the squeeze pressure to the entire area to be squeezed but to limit such pressure to an area against which rollers abut so that the squeeze time is shortened to render reduction in the percentage of water contained in the cake insufficient. Specifically, a problem has arisen in that there is insufficient pre-condensation of sludge since sludge is subjected to a heavy squeeze pressure at the first roller station and is thinly spread and this interferes with the effective dehydration.